Accurate noninvasive methods to measure cardiac output and arterial impedance parameters have been sought as an alternative to invasive catheter techniques with the associated patient discomfort and undesirable surgical procedure. Noninvasive techniques have been developed to predict arterial system impedance parameters which are based on electric circuit models of the arterial system. The circuit models are used in conjunction with pulse pressure waveform measurements on patients to predict cardiac output, arterial compliance, and so on.
The relationship between pressure and flow in the arterial system is similar to the relationship between voltage and current in an electrical system. Analogs between the electrical system and the physiological system have been proposed so that electrical circuits can be used to model the behavior of the physiological system. For example, electrical current is analogous to blood flow; each represents the movement of a substance (electrons, blood) through a closed system. In each system, the flow can be induced by a driving force, which is the voltage or pressure, respectively. The amount of flow in each system is determined by the impedance characteristics. For instance, capacitance is analogous to the elasticity (or compliance) of an artery. A narrowing of a blood vessel is analogous to increased resistance. In addition, the differential equations used to describe the behavior of electrical voltage and current can be directly applied to the physiological system.
Cardiac output may be expressed as the product of heart rate times volume of blood pumped per beat of the heart. Thus, under conditions of a consistent heart rate and stroke volume, EQU cardiac output (liters/min)=heart rate (beats/min).times.stroke volume (liters/beat).
Cardiac output or blood flow is also directly proportional to mean blood pressure and is inversely proportional to the peripheral resistance of the arterial system through which the blood flows.
Several noninvasive methods based on equivalent circuit model theory have been developed in the prior art. U.S. Pat. No. 4,993,420, entitled "Method and Apparatus for Noninvasive Monitoring Dynamic Cardiac Performance", to Welkowitz et al., and assigned to Rutgers University, the assignee herein, discloses a noninvasive method and apparatus employing two pulse transducers--one for measuring a carotid pulse waveform and the other for measuring a femoral pulse waveform. The carotid pulse waveform is applied as a voltage to a simulated aorta circuit, and the circuit component values are varied to develop a waveform output best matching the femoral pulse electrical waveform. The simulated circuit is then considered to be a representative model of the aorta of the living subject. An input current waveform corresponding to the aortic flow is calculated from the model. The circuit model used employs lumped circuit elements.
U.S. Pat. No. 5,101,828, entitled "Methods and Apparatus for Noninvasive Monitoring of Dynamic Cardiac Performance", to W. Welkowitz et al., and assigned to Rutgers University, discloses a similar noninvasive apparatus and method. Two pulse transducers are again employed to measure carotid and femoral pulse waveforms. Arterial parameters are predicted using a tapered aorta hybrid circuit model. This is a nonuniform hybrid model which provides a removable representation of the pressure transfer function of the aorta. By measuring input (carotid) and output (femoral) pressure waveforms, and finding the optimal fit between the model and arterial data, the cardiac output is estimated. A disadvantage of the above-cited methods is that they require two pulse waveform measurements at different locations--the carotid and the femoral artery positions. In clinical practice, measurement of the femoral arterial pulse is difficult. As a result, the above techniques suffer from being clinically impractical. In addition, measuring two pulse waveforms as opposed to only one waveform adds complexity to the measurement and to the associated measurement circuitry.
U.S. Pat. No. 5,211,177 entitled "Vascular Impedance Measurement Instrument", to Chesney et al., discloses a noninvasive vascular impedance instrument which apparently operates with the use of only one pulse pressure transducer. A finger-cuff transducer unit is employed to measure arterial blood pressure and provide waveform data which is digitized and applied to a microprocessor. A modified Windkessel circuit model of the arterial system is utilized to predict cardiac output and mean arterial pressure. This circuit model includes proximal (aortic) and distal (brachial) pulse pressure and proximal and distal arterial compliance. The accuracy reported in this patent for the disclosed method in comparison to a standard based upon a thermodilution or dye dilution technique appears to be satisfactory.
Another prior art method of predicting cardiac output is based on an unmodified Windkessel circuit model of the arterial system which consists of a single lumped element capacitor C to model arterial compliance during systole. By measuring heart rate (HR) and systolic minus diastolic pressure (PP), cardiac output is determined by: EQU CO(L/min)=HR(beats/rain).times.C(Farads).times.PP(mmHg).times.0.133.
Thus it is possible to obtain a measurement of cardiac output without having to decipher an arterial pressure waveform. However, it has been found that using a constant value for the capacitance C renders this method inaccurate. It would therefore be desirable to derive a general formula for C which substantially enhances the accuracy of this approach.
It is an object of the present invention to provide a noninvasive apparatus and method to accurately predict cardiac output and arterial system parameters which employs a single pulse pressure transducer, preferably to measure the carotid pulse pressure waveform.
It is a further object of this invention to provide such apparatus and method that models arterial compliance as a capacitance having one value during the systolic upshoot and another value during the diastolic decay of the pulse pressure waveform.
It is an additional object of this invention to provide such apparatus and method that measures a time varying arterial compliance during systole.
It is another object of the present invention to provide a method of measuring cardiac output from a measurement of a patient's heart rate and systolic minus diastolic blood pressure.